1. Field of the Invention
This invention relates to a golf club shaft.
2. Description of the Related Art
Generally, a shaft, used in a golf club, is produced by a method in which so-called prepreg sheets, each formed by arranging or orienting reinforcing fibers in one direction and then by impregnating these fibers with a synthetic resin, are wound in a superposed manner on a metal core, and then a cellophane tape is wound on this layer structure, and then the synthetic resin is thermally set or cured in a heating furnace, and then this product is cooled, and then the metal core and the cellophane tape are removed from this product, and then this product is subjected to other steps such as grinding and coating, thereby producing the golf club shaft.
Generally, the laminate structure, formed by the prepreg sheets wound on the metal core, comprises a body layer which comprises an oblique fiber layer formed by the prepreg sheets (that is, the oblique prepreg sheets having the reinforcing fibers oriented (that is, arranged to extend) in a direction oblique relative to the axis of the metal core (usually, at an angle of 45.degree. relative to the axial direction) ) wound in such a superposed manner that the direction of orientation of the fibers of the these prepreg sheets cross each other (usually at angles of .+-.45.degree. relative to the axial direction), and an axial fiber layer formed by the prepreg sheet (that is, the axial prepreg sheet having the reinforcing fibers oriented in the direction of the axis of the metal core). Further, reinforcing layers, formed respectively by an axial prepreg sheet and a peripheral prepreg sheet (having the reinforcing fibers oriented in a peripheral or circumferential direction), are provided at necessary portions of the body layer.
In the above construction, the oblique sheets and the axial sheet, used in the body layer, usually have a thickness of about 0.1 mm to about 0.2 mm.
However, as described above, the oblique sheets are wound on the metal core in such a superposed manner that the directions of orientation of the fibers of these sheets cross or intersect each other, and therefore the combined thickness of the oblique sheets are about twice larger than the thickness of the axial sheet. Namely, for example, if the oblique sheets and the axial sheet have a thickness of 0.1 mm, the combined thickness of the oblique sheets are 0.2 mm since the two oblique sheets are superposed together, and this thickness is twice larger than the thickness of the axial sheet. Therefore, when such thick sheet is wound, an uneven thickness condition due to this thickness develops at an overlapping portion at a winding-end region.
Particularly recently, shafts have been required to have a lightweight, high-elasticity design, and the number of winding of oblique sheets and axial sheets has been reduced. Therefore, if an uneven thickness condition due to the above-mentioned thickness difference develops at the end region of the wound sheets, there is encountered a problem that this causes the reduction of the strength, or the unevenness of the strength increases.
Usually, the shaft has a larger diameter at a grip side, and has a smaller diameter at a head side, and therefore a torsional rigidity is larger at the grip side, and is smaller at the head side. Therefore, the number of winding of the oblique sheets at the head side is different from that at the grip side so that the oblique fiber layer can have a larger thickness at the head side, and can have a smaller thickness at the grip side.
With this construction, however, there is encountered a problem that the strength is liable to be reduced at a generally-central portion of the shaft by a torsion and a load of the head, so that the shaft is liable to be broken or damaged.